1. Field of the Invention
The present invention relates to an apparatus and method for removing interference signal components between multiple transmitter antennas. More particularly, the present invention relates to an apparatus and method for removing interference signal components between multiple transmitter antennas when a mobile communication system with a plurality of transmitter and receiver antennas supports high-speed packet data.
2. Description of the Related Art
Current mobile communication systems are developing into wireless broadband communication systems of more than 5 GHz. For example, a wireless local area network (LAN) operates at 24 GHz of an industrial, scientific & medical (ISM) band corresponding to a frequency band capable of being used without a special license. In this case, the spectral efficiency of the system requires more than 10 bits/s/Hz per channel.
A multiple-input multiple-output (MIMO) system with a plurality of transmitter and receiver antennas is being considered as a mobile communication system capable of satisfying the spectral efficiency according to high-speed and high-capacity services.
FIG. 1 illustrates a structure of a conventional MIMO system.
When the number of antennas of a transmitter is M and the number of antennas of a receiver is N as illustrated in FIG. 1, the transmitter can simultaneously transmit independent data elements using the M antennas if a Vertical Bell Labs Layered Space-Time (V-BLAST) scheme is used. According to the V-BLAST scheme, the transmitter transmits parallel data and the receiver detects the transmitted data by repeatedly using a minimum mean-square error (MMSE) scheme. When the V-BLAST scheme is applied, an amount of data to be transmitted can be increased by the number of transmitter antennas.
When the MIMO system is used for the transmitter/receiver, additional degrees of freedom of the spatial dimension are introduced into the mobile communication system. In relation to the degrees of freedom, the MIMO system provides two space-time processing methods.
In a first method, space-time coding improves the reliability of a channel link. For example, the space-time coding system combats fading effect using diversity in a communication channel.
In a second method, spatial multiplexing improves the spectral efficiency. For example, the spatial multiplexing system can simultaneously transmit and receive high-speed data without additionally allocating bandwidth or power.
The conventional MIMO system has a technical limitation in that the number of transmitter antennas of a base station (BS) must be less than the number of receiver antennas of a mobile station (MS). However, there is a drawback in that the conventional MIMO system is not suitable for high-speed data communication due to the technical limitation associated with the number of transmitter antennas. As a result, the conventional MIMO system may degrade the spectral efficiency and system performance.
In the MIMO system, antennas need to be distinguished in a spatial domain. Accordingly, independent channel characteristics without any correlation are required. However, it is difficult for the MS to ensure the independent channel characteristics between the antennas when the number of receiver antennas is less than the number of transmitter antennas. When the number of receiver antennas is less than the number of transmitter antennas, the performance of the MS is significantly degraded.
For example, when the MIMO system of FIG. 1 uses the spatial multiplexing according to the second method, transmitter antennas 102, 104, and 106 simultaneously transmit different symbols S1, S2, . . . , SM. Accordingly, the transmitter antenna 102 transmits the symbol S1, the transmitter antenna 104 transmits the symbol S2, and the transmitter antenna 106 transmits the symbol SM.
When the number of transmitter antennas is the same as the number of receiver antennas as illustrated in FIG. 1, a multipath channel linearly operates in the MIMO system. When a frequency band is high, a propagation channel gradually becomes a line-of-sight channel and a correlation level increases. Accordingly, a problem occurs in actually implementing the MIMO system technology depending upon sufficient channel dispersion effect if the frequency exceeds 5 GHz. For example, a matrix coefficient of a MIMO channel is set to “1” in a non-multipath channel situation, and the channel cannot be applied to the spatial multiplexing.
A problem occurs in that a receiver using the MMSE system cannot operate when the matrix coefficient of a channel is small, and the receiver performance is severely degraded. As a result, significant performance degradation occurs due to interference between the antennas that cannot be negligible.
Accordingly, there is a need for an improved apparatus and method for removing interference signal components between transmitter antennas in order to improve signal detection performance from a receiver.